1. Field of the Invention
The present invention relates to a method and apparatus for evaluating a magnetic recording medium.
2. Description of the Related Art
Increasing recording densities of magnetic storage devices have caused the problem of so-called thermal stability of residual magnetization, or a decrease over time in the magnetization of bits recorded in a recording layer. Ferromagnetic materials with increased anisotropic magnetic field strength have been used for recording layers in order to increase the thermal stability of residual magnetization. This results in an increase in the recording magnetic field necessary for reversing the direction of the magnetization of the recording layer. However, increases in the maximum recording magnetic field that a magnetic head can generate have not kept up, so that the recording magnetic field may not be sufficient for recording.
In order to solve this problem, thermally assisted magnetic recording is proposed. (See, for example, Patent Document 1 listed below.) According to the thermally assisted magnetic recording, at the time of recording, a magnetic recording medium is heated by exposing the magnetic recording medium to laser light so as to reduce reversal magnetic field strength, thereby facilitating recording. According to the thermally assisted magnetic recording, the magnetization of the recording layer is reversed at high speed on the order of a nanosecond with the recording layer being exposed to laser light, and it is necessary to design a medium that is best suited to the conditions and magnetization behavior of the recording layer.
According to the thermally assisted magnetic recording, however, the temperature rises to 100° C. to several hundred ° C. in an extremely short time, so that it is extremely difficult to evaluate the magnetic properties of the magnetic recording medium. Further, it is necessary to observe an area of the magnetic recording medium that is less than or equal to approximately 100 nm on a side.
The magnetization condition at the time of laser light exposure is measured using a so-called laser SQUID (Superconducting Quantum Interference Device) microscope. (See, for example, Non-Patent Document 1 listed below.)
Further, high-speed magnetization reversal is measured by, for example, exposing a sample disposed in a magnetic field to laser light of an extremely short duration (Non-Patent Document 2 listed below), applying a magnetic field of an extremely short duration to a sample and measuring magnetic reversal using the Kerr effect (Non-Patent Document 3), or using an electron beam.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2005-222669
[Non-Patent Document 1] Daibo, M.; “Laser SQUID Microscope for Semiconductor Testing,” Journal of the Magnetics Society of Japan, 29, No. 1, 14-19 (2005)
[Non-Patent Document 2] van Kempen, M. et al.; “All-Optical Probe of Coherent Spin Waves,” Physical Review Letters, 88, No. 22, 227201-1-227201-4 (2002)
[Non-Patent Document 3] Back, C. H. et al.; “Magnetization Reversal in Ultrashort Magnetic Field Pulses,” Physical Review Letters, 81, No. 15, 3251-3254 (1998)
However, according to Non-Patent Document 1, there are problems in that it is difficult to detect fine magnetization behavior because the resolution of the laser SQUID microscope is as large as approximately several μm and that a large amount of money is required for equipment such as a large-scale cooling system in order to measure a large number of magnetic recording media.
Further, according to Non-Patent Document 2 or 3 or a measurement method using an electron beam, there is a problem in that it is difficult to measure a large number of magnetic recording media in a simple manner because the main purpose is to observe high-speed reversal of magnetization, which requires an expensive and large-scale system. Further, introduction of such a system as a testing device into the process of manufacturing magnetic recording media is difficult in terms of costs.